The ceramic circuit board has lately attracted considerable attention because of its easiness of multi-layer interconnection and for its thermal expansion coefficient close to that of silicon of IC chips and because inner via conductors can be formed. These features of the ceramic circuit board contribute to increasing the packaging density of, for example, semiconductor ICs. Therefore, the ceramic circuit board now finds many applications in, for example, camera-built-in video systems with smaller circuits.
FIG. 3 cross-sectionally shows a ceramic circuit board which is formed using a conventional gold paste.
An IC bare chip is mounted by a well-known mounting technique called COB (Chip-On-Board) on a surface of the ceramic circuit board of FIG. 3. In COB, IC bare chips are directly mounted on ceramic circuit boards. With the continuing miniaturization of electronic circuit components, this COB mounting technique plays an important role. The COB mounting technique is suitable for multi-layer interconnection.
FIG. 3 shows a conventional ceramic circuit board using a usual gold paste as gold wiring material. This prior art board comprises a ceramic board 11, an IC bare chip 15, gold wiring 17, a gold wire 19, and a silver paste 20. The gold wiring 17, formed by subjecting a usual gold paste to a firing process in the air at temperatures of 800.degree. C. to 900.degree. C., is electrically connected by the gold wire 19 to the IC bare chip 15 that is mounted on the ceramic board 11 through the silver paste 20.
FIG. 4 shows another conventional ceramic circuit board using a usual gold paste as gold wiring material. In the ceramic circuit board of FIG. 4, face-down bonding technique is employed, which needs less area for connecting the IC bare chip 15 and the gold wiring 17 compared to a technique used in the ceramic circuit board of FIG. 4 so that a higher packaging density can be accomplished. FIG. 4 shows a gold bump 16 and a joint layer 21. The IC bare chip 15 carrying thereon the gold bump 16 having a diameter of about 0.2 mm is formed on the gold wiring 17 obtained by sintering a usual gold paste in the air at 800-900.degree. C. so as to establish connection between the gold bump 16 and the gold wiring 17 through the joint layer 21. The joint layer 21 may be an electroconductive adhesive, an anisotropic electroconductivity film/adhesive, a solder, or the similar material. This mounting technique, compared to a face-down bonding technique in which a solder bump is used, is able to provide improved circuit boards having a higher reliability against, for example, heat shock.
The above-described techniques, in which the IC bare chip 15 is mounted on the gold wiring 17 by the gold wire 19 or the gold bump 16, have been regarded as a most reliable IC bare chip mounting technique so far, since gold can stand chemical changes such as oxidization.
Every circuit wiring must be formed thin to meet the high-density requirement. Copper is attractive as a wiring material because of its low resistivity and less migration tendency, and use of copper contributes to speeding up the transfer of signals.
Depicted in FIG. 1 is an improved ceramic circuit board that is fabricated using advantages of both of gold wiring and copper wiring. More specifically, gold is used at a connecting part between an IC bare chip and a ceramic board, while on the other hand copper is used for other interconnections.
FIG. 1 shows surface-layer copper wiring 12, a solder layer 13, a chip component 14, a copper/gold overlap layer 18, a copper via conductor 22, and inner-layer copper wiring 23.
The wiring of the ceramic circuit board of FIG. 1 may be formed as follows. The surface-layer copper wiring 12, the copper via conductor 22, and the inner-layer copper wiring 23 are formed on the ceramic board 11. A gold paste is transferred onto an electrode on the ceramic board 11 where the IC bare chip 15 is placed. Following a drying process, the gold paste thus transferred undergoes a binder burn-out process (removal of a binder) and a firing process in a belt furnace in a nitrogen atmosphere. As a result, the gold wiring 17 and the copper/gold overlap layer 18 are formed. The reason for firing the gold paste in a nitrogen atmosphere is to protect the surface-layer copper wiring 12, the copper via conductor 22, and the inner-layer copper wiring 23 from oxidization.
Usual gold pastes for ceramic circuit boards are suitable when fired in the air. However, when a usual gold paste is fired together with the surface-layer copper wiring 12, the copper via conductor 22, and the inner-layer copper wiring 23, this causes the copper of each wiring to oxidize, therefore dramatically increasing the wiring resistance. As a result, the surface-layer copper wiring 12, the copper via conductor 22, and the inner-layer copper wiring 23 become unworkable.
A solution to the above-described problem may be obtained by firing a gold paste in an atmosphere of nitrogen; however, the obtained gold wiring comes to have not only a higher resistivity but also a weaker adhesion strength to a ceramic board, as compared to one obtained by firing carried out in the air. Particularly, the decrease in gold wiring-to-ceramic board adhesion strength is a serious problem.
Another type of gold paste has been tried, which contains, as its organic binder, only a resin capable of decomposing and vaporing in an atmosphere of nitrogen (e.g., polymethyl acrylate). Such a gold paste was fired in a nitrogen atmosphere, and the resulting gold wiring had been found to fail to improve the adhesion strength to the ceramic board.
A different approach may be tried. In other words, the surface-layer copper wiring 12 is formed after a usual gold paste has been fired in the air to form gold wiring. This approach, however, is applicable only to the manufacture of ceramic circuit boards having neither the copper via conductor 22 nor the inner-surface copper wiring 23.
Another approach may be tried. In this approach, the IC bare chip 15 is connected to the surface-layer copper wiring 12 which is gold-plated. This approach, however, is expensive and causes to the surface-layer copper wiring 12 a problem of how to resist such plating.